1. Field of the Invention
The present invention relates to an image forming apparatus such as a color copying machine or a color printer having a plurality of developing devices, each of which can develop an electrostatic latent image formed on an image carrying member corresponding to the developing device with developer of a color assigned to the developing device.
The present invention also relates to a developing method in an image forming apparatus having a plurality of developing devices.
2. Description of Related Art
Nowadays, image forming apparatuses such as copying machines and printers, which form images by developing electrostatic latent images formed on image carrying member with developer, are widely available, and color image forming apparatuses are rapidly spreading. In this situation, awareness of users about the image quality and image formation cost keep on rising.
In connection with the image quality, it is desired to eliminate or minimize image noises in the images. So-called “fogging” is a kind of such image noises. The “fogging” phenomenon is primarily caused by lowering of a charge amount of the developer used for image formation, and this lowering is caused by deterioration of the developer.
The lowering of the developer charge amount due to the deterioration of the developer causes the above fogging, and further increases a developer consumption due to occurrence of the fogging so that a cost to the user increases.
The developing devices developing the electrostatic latent images on the image carrying member can be roughly divided according to the type of the developer used therein into two types, i.e., a two-component developing device using a so-called two-component developer primarily formed of toner and carrier, and a one-component developing device using a so-called one-component developer which is primarily formed of toner and does not contain carrier.
In either of the above types, the developing device generally employs a developing roller, which carries the developer on its peripheral surface, and transfers it to a developing region. When developing the electrostatic latent image, a development bias voltage is generally applied to the developing roller for smoothly moving the developer on the developing roller to the electrostatic latent image. For obtaining an image of high quality by suppressing unremovable adhesion of the developer onto the developing roller, the development bias voltage is often formed of an AC development bias voltage employing a combination of DC and AC voltages (typically, a DC voltage and an AC voltage superimposed thereon).
In the developing device (one-component developing device) using the developer primarily formed of the toner, a charging member is generally in contact with the developer, which is carried on the developing roller and is being transferred to the developing region, and frictionally charges the developer for obtaining a charge amount of the developer required for development of the electrostatic latent image. The charging member generally serves also as a developer restricting member for restricting the amount of developer transferred to the developing region to provide a thin layer thereof.
However, if the charging is performed only by the frictional charging, a failure occurs in developer charging when the developer deteriorates due to repeating of the image formation. Therefore, such a manner is also employed that the developer is electrically charged by setting a potential difference between a developer charging member also serving as the above restricting member and the developing roller, or by setting a potential difference between a developer charging member, which is independent of the developer charging member also serving as the restricting member, and the developing roller.
It is preferable to produce the potential difference between the developer charging member and the developing roller by applying an AC charge bias voltage for suppressing unremovable or strong adhesion of the developer onto the developing roller or the developer charging member.
In connection with this, Japanese Laid-Open Patent Publication No. 2001-109243 (JP 2001-109243 A) has disclosed:
(1) a developing device which employs an AC development bias voltage formed of a DC voltage and an AC voltage superimposed thereon as a development bias voltage to be applied to the developing roller, and particularly, a developing device which applies an AC charge bias formed of another DC voltage and an AC voltage superimposed thereon to the developer charging member for producing a potential difference between the developer charging member also serving as the restricting member and the developing roller, and
(2) a developing device generating an AC charge bias voltage, of which phase on only one side (i.e., positive or negative side) is offset (i.e., of which amplitude is reduced) by using a Zener diode, and applying it to the developer charging member for obtaining the AC charge bias voltage applied to the developer charging member at a low cost.
FIG. 10(A) shows an example of a bias voltage applying circuit in the latter developing device.
According to the circuit shown in FIG. 10(A), a power source formed of a DC power source Vdc and a rectangular-wave AC power source Vac, which are connected in series, are employed as an AC development bias power source pw, and a Zener diode td″ and a resistance element r″ are connected in series to an output terminal a″ of the power source pw. A developing roller is connected to the output end a″, and a charging member is connected to a contact b″ between the Zener diode td″ and the resistance element r″.
This circuit is an example of a circuit for image formation in which a negatively chargeable photosensitive member is employed as an image carrying member for forming an electrostatic latent image thereon, and reversal development of the electrostatic latent image on the photosensitive member is performed with negatively chargeable toner.
In this circuit, it is assumed that the DC power source Vdc of the development bias power source pw provides an output voltage of −400 V, a peak-to-peak voltage of the AC power source Vac is 1800 V and a breakdown voltage of the Zener diode td″ is 200 V. In this case, the developing roller is supplied with the AC development bias voltage having a waveform represented by solid line in FIG. 10(B). The charging member is supplied, as represented by broken line in FIG. 10(B), with an AC charge bias voltage which has a phase synchronized with the AC development bias voltage applied to the developing roller, and exhibits an offset potential difference of 200 V in the phase on the positive side. In the phase on the negative side, the substantially same potentials are applied to the developing roller and the charging member.
This circuit structure utilizes the AC power source Vac in the AC development bias power source pw, and thereby can provide the AC charge bias voltage at a low cost. The AC charge bias in the phase on the positive side is offset with respect to the development bias (i.e., the offset potential difference is set) so that it is attempted to optimize the toner charge amount.
In any one of the conventional developing devices already described, it is possible to increase the charge level of the deteriorated developer, and thereby to reduce the reversely chargeable developer, i.e., the developer which is charged reversely to the regularly chargeable developer, and therefore becomes a primary cause of the fogging phenomenon. Therefore, the image quality can be improved.
According to the study by the inventors, a developing device configured to set a potential difference between a developer charging member and a developing roller causes such a situation that initial developer, which has a charging property at an appropriate level and is usually in a low-humidity environment, is excessively charged.
The developing roller strongly restrains the excessively charged developer by a Coulomb force, and suppresses smooth movement thereof toward an electrostatic latent image on an image carrying member so that desired development cannot be performed. Further, excessively charged components may cause fogging.
For overcoming the above problem, the potential difference between the charging member and the developing roller may be reduced. Although such setting can overcome the above problem relating to the initial developer, it is difficult to prevent the occurrence of fogging due to the deteriorated developer by such setting.
In an image forming apparatus such as a color copying machine or a color printer, which employs a plurality of developing devices corresponding to respective colors of developer, each developing device may have a circuit structure which applies to a developer charging member a charge bias voltage having a phase synchronized with an AC development bias voltage and exhibiting an offset potential difference with respect to this AC development bias voltage. However, each of the developing devices independently suffers from a problem similar to that already described. Thus, variations in factors determining the charging property of the developer cannot be dealt with, and a failure occurs in developer charge amount.
In an image forming apparatus with a plurality of developing devices, a developer charging member may be supplied with an AC charge bias voltage, which is formed of a DC voltage and an AC voltage superimposed thereon, and is independent of an AC development bias voltage applied to a developing roller, as disclosed in Japanese Laid-Open Patent Publication No. 2001-109243.
According to this configuration, the amplitude of the AC development bias voltage can be increased independently of the AC charge bias voltage so that the movability of the developer from the developing roller to an electrostatic latent image carrying member can be improved, and in other words, the developing property can be improved. However, the charge bias is fixed with respect to the development bias, and therefore it is impossible to optimize the developer charge amount in accordance with variation in a factor determining the chargeability of the developer.
Further, each developing device requires two AC power sources, i.e., an AC power source for the AC development bias voltage and an AC power source for the AC charge bias voltage. Since the AC power sources are expensive, the image forming apparatus becomes expensive.
The inventor has made study to found the following.
The circuit structure shown in FIG. 10(A) already described was employed to apply an AC charge bias voltage to the charging member while selectively using various Zener diodes having different breakdown voltages, and thereby changing the offset potential difference to various values, respectively. Thereby, the following was found in connection with the case where initial toner (i.e., unused toner) is used in a low humidity environment, and thus charging is relatively easy, and in connection with the case where the toner (used toner) is used in a high humidity environment, and thus charging is relatively difficult.
As illustrated in FIG. 9, (1) the offset potential difference governs the toner charge amount in both the cases, and (2) there is a tendency that the charge amount of the initial toner is larger than that of the user toner. For example, if the appropriate charge amount of the toner is in a range from −20 μC/g to −40 μC/g, the offset potential difference, which can achieve the charge amount of the used toner within a range from −20 μC/g to −40 μC/g, achieves the toner charge amount of the initial toner exceeding −40 μC/g. Further, it was found that the appropriate developer charge amount is obtained by controlling the offset potential difference according to a factor affecting the charging property of the developer.